CVE-2026-25133: October CMS Stored XSS Vulnerability

CVE-2026-25133 Overview

October CMS, a popular Content Management System and web platform built on the Laravel framework, contains a stored cross-site scripting (XSS) vulnerability in versions prior to 3.7.14 and 4.1.10. The vulnerability exists in the SVG sanitization logic, where a regex pattern used to strip event handler attributes (such as onclick or onload) can be bypassed using crafted payloads that exploit how the pattern matches attribute boundaries. This allows malicious SVG files with embedded JavaScript to be uploaded through the Media Manager.

Critical Impact

Successful exploitation could lead to privilege escalation if a superuser views or embeds the malicious SVG file, potentially allowing attackers to execute arbitrary JavaScript in the context of an administrative session.

Affected Products

• October CMS versions prior to 3.7.14 (v3.x branch)
• October CMS versions prior to 4.1.10 (v4.x branch)
• Installations with backend access and media upload permissions enabled

Discovery Timeline

• 2026-04-14 - CVE CVE-2026-25133 published to NVD
• 2026-04-14 - Last updated in NVD database

Technical Details for CVE-2026-25133

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw resides in October CMS's SVG file sanitization mechanism within the Media Manager component.

SVG files are XML-based vector image formats that can contain embedded JavaScript through event handler attributes like onclick, onload, onmouseover, and others. October CMS implements a regex-based sanitizer to strip these potentially dangerous attributes before allowing SVG uploads. However, the regex pattern contains a boundary-matching weakness that allows attackers to craft payloads that slip through the sanitization process.

The attack requires authenticated backend access with media upload permissions, making it an authenticated attack vector. However, the potential for privilege escalation to superuser accounts significantly increases the risk. When a privileged user views or embeds the malicious SVG file, the embedded JavaScript executes in their browser context, potentially exposing session tokens, enabling CSRF attacks, or allowing full account takeover.

Root Cause

The root cause is an insufficient regex pattern used for stripping event handler attributes from SVG content. The regex fails to properly account for all valid attribute boundary conditions, allowing attackers to structure their payloads in ways that bypass the pattern matching. This is a common pitfall when using regex for HTML/XML sanitization rather than proper DOM-based parsing and allowlist approaches.

Attack Vector

The attack is network-based and requires:

1. Authenticated access to the October CMS backend
2. Media upload permissions to access the Media Manager
3. Social engineering or natural workflow to get a superuser to view or embed the malicious SVG

An attacker with lower-level backend privileges can upload a specially crafted SVG file through the Media Manager. The malicious SVG contains JavaScript disguised to bypass the regex sanitizer. When the SVG is later viewed directly, embedded in a page, or previewed by a superuser, the JavaScript payload executes in their browser context.

The vulnerability allows crafted SVG files to embed malicious JavaScript by exploiting weaknesses in regex-based attribute filtering. Attackers can structure event handler attributes in ways that don't match the expected boundary patterns. For detailed technical information about the specific bypass technique, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-25133

Indicators of Compromise

• Unusual SVG files uploaded through the Media Manager containing obfuscated or encoded content
• SVG files with unexpected event handler attributes or <script> tags that appear malformed
• Backend access logs showing media uploads from accounts that don't typically upload SVG content
• Reports of unexpected JavaScript behavior when viewing media assets

Detection Strategies

• Implement file integrity monitoring on the media upload directory to detect suspicious SVG uploads
• Review web application firewall (WAF) logs for SVG file uploads containing JavaScript-related keywords
• Monitor backend user activity for unusual patterns of media uploads, especially from low-privilege accounts
• Conduct regular security scans of uploaded media assets for embedded scripts

Monitoring Recommendations

• Enable detailed logging for Media Manager upload activities
• Configure alerts for SVG file uploads, particularly from accounts that rarely upload media
• Implement content security policy (CSP) headers to mitigate JavaScript execution from untrusted sources
• Regularly audit media storage directories for suspicious file content

How to Mitigate CVE-2026-25133

Immediate Actions Required

• Upgrade October CMS to version 3.7.14 or later for v3.x installations
• Upgrade October CMS to version 4.1.10 or later for v4.x installations
• Audit existing SVG files in the media library for potentially malicious content
• Temporarily restrict SVG upload permissions to trusted users until patching is complete

Patch Information

The October CMS maintainers have addressed this vulnerability in versions 3.7.14 and 4.1.10. The fix improves the SVG sanitization logic to properly handle attribute boundary matching, preventing the regex bypass technique. Organizations should update to these versions immediately through their standard Composer update process. For more details, see the GitHub Security Advisory.

Workarounds

• Disable SVG file uploads entirely through the Media Manager configuration until patching is possible
• Implement a web application firewall (WAF) rule to block SVG uploads containing event handler patterns
• Restrict media upload permissions to only highly trusted backend users
• Use a CDN or separate domain for serving uploaded media to contain potential XSS impact

If immediate patching is not possible, consider modifying the allowed upload file types in October CMS configuration:

# config/media.php - Disable SVG uploads temporarily
'allowed_extensions' => 'jpg,jpeg,png,gif,webp,pdf,doc,docx',
# Remove 'svg' from the allowed list until patch is applied